Has anyone had a issue with reserve mileage distance changing? Mine has went rom
several miles to just a couple.can the petcock cause this?

 

I would say yes.It would not hurt to remove it and check it.
You could also let the the gas drain out when in reserve and see if there is any fuel left in the tank.

 

How do you check it and what do i look for? don't really know how they are designed.it look's like there is just one tube from the picture's i've seen.

 

I was thinking there may be a filter screen on the end of the tube that could be plugged and not letting the gas in at the lowest level..Usally there are 2 tubes set at different heights,one for the main and the lower for the reserve.I could be wrong.
I would let the fuel drain out of the reserve,when it stops flowing see if there is any fuel left in the tank.
There could also be something wrong with the petcock when it is in the reserve position.

 

I'll check and let you know sometime this week. Thank's.